The invention of the subject application pertains to an induction logging apparatus for oil well boreholes and more particularly to a multifrequency signal transmitter associated with the induction logging apparatus for generating a multifrequency output signal that induces in the formation eddy currents corresponding to each frequency of the multifrequency signal.
An induction logging apparatus disposed in the borehole of an oil well basically comprises at least one transmitter and at least two receiving coils mounted on a support and axially spaced from each other in the direction of the borehole. The transmitting coil is energized by an alternating current at a frequency which is typically 20 KHz and generates an electromagnetic field which induces in the formation surrounding the borehole eddy currents which flow coaxially to the borehole, the intensity of which is proportional to the conductivity of the formation. The field generated in turn by these eddy currents induces in the receiving coil an electromotive force (EMF), which produces a received signal in the receiving coil. By suitably processing the received signal from the receiving coil, a measurement of the conductivity of the formation is obtained.
The transmitter associated with prior art induction tools produced a signal at a single frequency. This single frequency signal induced eddy currents in the surrounding formation corresponding to the single frequency of the transmitted signal. While a corresponding received signal from the receiving coil may be processed, the amount of information corresponding to the conductivity of the formation, derived from the processing of the single frequency signal, is limited. If a multi-frequency signal transmitter is developed, which generates a multi-frequency signal and which induces multiple eddy currents in the surrounding formation, much more information may be derived. The additional information, when used with suitable signal processing techniques, can be used to obtain improved estimates of the earth's conductivity.